git-annex/doc/design/assistant/telehash.mdwn

[Telehash](http://telehash.org/) for secure P2P communication between
git-annex (assistant) repositories.

## telehash implementation status

* node.js version seems almost complete
* C version currently lacks channel support and seems buggy (13 Jan 2014)
* No pure haskell implementation of telehash v2. There was one of
  telehash v1 (even that seems incomplete). I have pinged its author
  to see if he anticipates updating it.
* Rapid development, situation may change in a month or 2.
* Is it secure? A security review should be done by competant people
  (not Joey). See <https://github.com/telehash/telehash.org/issues/23>
* **Haskell version** 
  <https://github.com/alanz/htelehash/tree/v2/src/TeleHash>
  Development on v2 in haskell is just starting up!

## implementation basics

* Add a telehash.log that maps between uuid and telehash address.
* On startup, assistant creates a new telehash keypair if not already
  present; stores this locally and generates a telehash address from it,
  stored in telehash.log.
* Use telehash for notifications of changes to the repository
* Do git push over telehash. (Pretty easy, may need rate limiting in
  situations involving relays.)
* Remove git push over XMPP (which has several problems including
  XMPP being an unreliable transport, requiring a separate XMPP account per
  repo, and XMPP not being end-to-end encrypted)

## telehash address discovery

* Easy way is any set of repos that are already connected can communicate
  them via telehash.log.
* Local pairing can be used for telehash address discovery. Could be made
  to work without ssh (with content transfer over telehash discussed
  below).
* XMPP pairing can also be used for telehash address discovery. (Note that
  MITM attacks are possible.) Is it worth keeping XMPP in git-annex just
  for this?
* Telehash addresses of repositories can be communicated out of band (eg,
  via an OTR session or gpg signed mail), and pasted into the webapp to
  initiate a repository pairing that then proceeds entirely over telehash.
  Once both sides do this, the pairing can proceed automatically.

## content transfer over telehash

* In some circumstances, it would be ok to do annexed content transfer
  over telehash.  
  Need to check if there are MTU problems with large data bodies in
  telehash messages.  
  Probably not when a bridge is being used, due to required rate
  limiting in bridging over telehash. Cloud transfer remotes still needed for
  those situations.
* On a LAN, telehash can be used to determine the current local IP address
  of another computer on the LAN. The 2 could then determine if either uses
  ssh and if so use regular git-annex-shell for transfers. Or could do
  annexed content transfer directly over telehash.

## generic git-remote-telehash

This might turn out to be easy to split off from git-annex, so `git pull`
and `git push` can be used at the command line to access telehash remotes.
Allows using general git entirely decentralized and with end-to-end
encryption.

## separate daemon?

See [[git-remote-daemon]] for a design.

Advantages:

* `git annex sync` could also use the running daemon
* `git-remote-telehash` could use the running daemon
* c-telehash might end up linked to openssl, which has licence combination
  problems with git-annex. A separate process not using git-annex's code
  would avoid this.
* Allows the daemon to be written in some other language if necessary
  (for example, if c-telehash development stalls and the nodejs version is
  already usable)
* Potentially could be generalized to handle other similar protocols.
  Or even the xmpp code moved into it. There could even be git-annex native
  exchange protocols implemented in such a daemon to allow SSH-less
  transfers.
* Security holes in telehash would not need to compromise the entire
  git-annex. gathd could be sandboxed in one way or another.

Disadvantages:

* Adds some complexity.